The present invention relates generally to control systems, and more particularly to model predictive control employing novel techniques for optimizing the blending of denaturants with undenatured biofuels.
Many processing applications, such as biofuel production, include sub-processes where multiple fluids may be blended. For example fuel-grade ethanol (denatured) has industry specifications. These specifications include specific gravity, water mass percentage, ethanol percentage, sulfur content, and % hydrocarbon denaturant. Hydrocarbon denaturant is tested to an ASTM D-4806 standard and this hydrocarbon denaturant has a maximum and a minimum specification. Typical ranges for ASTM D-4806 are from 1.96-4.76 volume % of hydrocarbon. In addition, the water specification ASTM E-203 is less than 0.82 mass % of water.
The ASTM D-4806 standard is a specification in relation to anhydrous denatured fuel ethanol intended for blending with unleaded or leaded gasolines for use as a spark-ignition automotive engine fuel. The only denaturants allowed for fuel ethanol are natural gasoline, gasoline components, or unleaded gasoline at the minimum concentration prescribed. Gasoline and product ethanol prices are known to vary substantially in the market. The price spread between the two blend components is a variable which fuel grade ethanol manufacturers consider when denaturing fuel ethanol.
With water content being an allowable tolerance and the knowledge that higher water contents in fuel grade ethanol lower the cost of operation, blenders tend to run moisture at the specification limit (e.g., 0.82 mass % water). Producers may perform a blend calculation to ensure that the final blend meets ASTM D-4806 specifications and is within or at the ASTM E-203 limit to maximize blending profit. In other words the volume % blending of ethanol and gasoline parts may be done within the water mass % specification of the final fuel grade ethanol. This may be done off-line by most biofuels manufacturers by mathematical relationships of volumetric mixing rules of fluid mixtures. However with known measurements of desired water content and the cost of ethanol and gasoline, this may also be accomplished using model predictive control (MPC) techniques, integrating the blending requirements with the adjustment of operating targets of molecular sieve process units.
E-grade denatured fuel ethanol or a customer's specifications are required. One such process is the blending of denaturants with biofuels, which may be done in order to ensure that the biofuels are not fit for human consumption, thereby preventing people from drinking the biofuels. However, in many such applications, when controlling the blending of the multiple fluids, a fixed set point for the percentage of one fluid within another fluid (e.g., of a denaturant within a biofuel) may often be used. Therefore, blending of the multiple fluids may be performed in a constant manner based on this fixed set point, as opposed to dynamically optimizing the blending percentages of the fluids based upon final biofuel product specifications and the blending economics of the blending components.